1. Field of the Invention
The present invention relates to a thermal head which can be suitably used for printers, facsimile machines, copying machines, etc., of a thermo-sensitive or thermo-transfer type.
2. Description of the Related Art
Various types of thermal heads have been proposed and practically used for printers, facsimile machines, copying machines, etc., of a thermo-sensitive or thermo-transfer type. For example, a flat type thermal head is known in which a substrate has a printing surface which is formed thereon with a resistance-heating section and arranged co-planar with a surface region provided with a driver IC for driving the heating section. An end-face type thermal head is also known in which a resistance-heating portion is provided on the end-surface of a substrate which is perpendicular to a plane provided therein with a driver IC. In many cases, the arrangement of the printing surface of known thermal heads is such that alumina or the like ceramic substrate having a grazed layer is provided thereon with a heating element in the form of an array of a plurality of resistance bodies. The heating element is provided with electrodes to be connected to an electric power source and covered by a protective surface layer for preventing a premature wear of the resistance-heating element, and such surface layer is typically composed of silicon nitride or glass. In this connection, reference may be had to "Handbook for Recording/Memory Technology" edited by Kokado et al, and published on Aug. 25, 1992 by Maruzen Publisher, Japan, pages 328-329, the disclosure of which is incorporated herein by reference.
With the known thermal heads discussed above, for successively printing information on information carrier, e.g., a sheet of thermo-sensitive paper or ordinary paper, the information carrier and possibly an ink ribbon film is brought into pressure contact with the thermal head by a platen roller and caused to slide over the resistance-heating element in tangential or circumferential direction of the platen roller, with the protective surface layer between the information carrier and the heating element. In this instance, the application of pressure by the platen roller is to ensure that the heat generated by the resistance-heating element on the head substrate is effectively utilized for thermal print recording of information.
Although the protective surface layer on the printing surface does serve to provide the resistance-heating element with a certain degree of durability, formation of the protective surface layer not only requires complicated production steps and a relatively high manufacturing cost, but also makes it difficult to increase the printing speed due to deterioration with time in the thermal characteristic of the head. Moreover, because the thermal head is continuously maintained in a sliding pressure contact with the information carrier during operation, the durability of the protective layer tends to be insufficient particularly in the case of a heavy duty use as in a bar-code printer. Such insufficient durability often results in damage to the resistance-heating element and malfunctions of the thermal head, e.g. lack and/or dislocation of dots which form a desired code or character to be printed.